1. Field of the Invention
The present invention relates to a kinetic pressure fluid bearing apparatus, and more particularly, to a kinetic pressure fluid bearing apparatus in which a clearance supporting portion is formed between a rotary shaft and a thrust bearing which faces the rotary shaft.
2. Description of the Related Art
Recently, with improvements in the fields of information and computer technology, there has arisen a need for rotary shafts with higher rotational speed and high accuracy, but with no unwanted movement or oscillation. Such rotary shafts are needed in driving motors for various machines, such as a polygon mirror driving gear of a laser printer, a spindle motor of a hard disk, a head driving motor of a VCR, and the like. Movement or oscillation of the rotary shaft will degrade the performance of the machine. A driving motor capable of stably rotating at high speed has been developed together with a fluid bearing apparatus which enables the rotary shaft of the driving motor to rotate at high speed with high accuracy.
A conventional fluid bearing apparatus which is applied to the machines requiring high accuracy and high speed will be described with reference to FIG. 1, which shows a sectional view illustrating a fluid bearing apparatus and a distribution curve of pressure occurring in a rotary shaft by a thrust bearing.
Referring to FIG. 1, a thrust bearing 50 is positioned in one end of a substrate 10 and mounted to the substrate 10 and to a sleeve 20. A rotary shaft 30 is inserted into a through hole 25 of the sleeve 20 to face a surface of the thrust bearing 50. The rotary shaft 30 supports its vertical thrust load. A first kinetic pressure generating groove 50a is formed on the surface of the thrust bearing 50. An air vent 20a is formed in the sleeve 20 to implant a fluid into a portion where the thrust bearing 50 and the rotary shaft 30 face each other. A herring bone shaped second kinetic pressure generating groove 30a is formed on either an external side of a part of the rotary shaft 30 which is inside the opening 25 of the sleeve 20 or an internal side of the sleeve 20 within the opening 25.
The operation of the fluid bearing apparatus as aforementioned will be described below.
When the power is applied to the apparatus, the rotary shaft 30 starts to rotate. The rotational speed of the rotary shaft 30 increases to a predetermined value. A vortex flow having the same direction as the rotational direction of the rotary shaft 30 is then formed at a lower end portion of the rotary shaft 30. The vortex flow flows into edge portions A and C of the first kinetic pressure generating groove 50a. Then, the vortex flow rotates and flows into a center portion B of the first kinetic pressure generating groove 50a. The area of the first kinetic pressure generating groove 50a is gradually reduced as one goes from the edge portions A and C towards the center portion B. A low fluid pressure occurs at the edge portions A and C of the first kinetic pressure generating groove 50a. A minimum fluid pressure, which boosts the rotary shaft 30, occurs at the center portion B. At this time, to boost the rotary shaft 30 from the first kinetic pressure generating groove 50a, the fluid pressure has to be higher than tare and load of the rotary shaft 30 which drops the rotary shaft 30.
When the fluid pressure relative to the rotational speed of the rotary shaft 30 becomes higher than the tare and load of the rotary shaft 30, the rotary shaft 30 is boosted from the first kinetic pressure generating groove 50a to rotate without contacting with the first kinetic pressure generating groove 50a. 
The occurrence of such a fluid pressure in the first kinetic pressure generating groove is determined by the rotational speed of the rotary shaft, the area of the first kinetic pressure generating groove, and the clearance between the rotary shaft and the first kinetic pressure generating groove. Since the area of the first kinetic pressure generating groove and the clearance are determined in advance by the tare and load of the rotary shaft, the fluid pressure finally depends on the rotational speed of the rotary shaft.
In the conventional fluid bearing apparatus, the rotary shaft 30 reaches a predetermined number of revolutions per minute (rpm) in a predetermined time after the rotary shaft started to rotate. In this case, the fluid bearing apparatus has several problems.
In the case that the fluid pressure in the first kinetic pressure generating groove is lower than the tare and load of the rotary shaft, the rotary shaft rotates in contact with the first kinetic pressure generating groove. This results in abrasion and overload of the rotary shaft and the first kinetic pressure generating groove due to friction.
In addition, when the fluid pressure becomes similar to the tare and load of the rotary shaft as the rotational speed increases, the rotary shaft repeats to boost from the first kinetic pressure generating groove under the circumstances that it is contacted with the first kinetic pressure generating groove. This results in movement and oscillation of the rotary shaft.
Such problems occur also in the case that the rotary shaft slows to a stop.
Accordingly, the present invention is directed to a kinetic pressure fluid bearing apparatus that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.
An object of the present invention is to provide a kinetic pressure fluid bearing apparatus capable of preventing abrasion and overload of a rotary shaft and a first kinetic pressure generating groove, which would otherwise occur when the rotary shaft starts and stops rotating.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, a kinetic pressure fluid bearing apparatus according to the present invention includes a sleeve mounted onto a bearing bracket, a rotary shaft inserted into a through hole of the sleeve, having a second kinetic pressure generating groove on an external side which faces an internal side of the through hole, a thrust bearing mounted into one end of the through hole of the sleeve, having a first kinetic pressure generating groove on its surface which faces a lower end portion of the rotary shaft, and a clearance supporting portion formed between the first kinetic pressure generating groove on the surface of the thrust bearing and the lower end portion of the rotary shaft.
In one embodiment of the present invention, the clearance supporting portion includes a ball inserting groove formed on the lower end portion of the rotary shaft and a ball inserted into the ball inserting groove.
In another embodiment of the present invention, the clearance supporting portion includes a ball inserting groove formed on the surface of the thrust bearing and a ball inserted into the ball inserting groove.
In other embodiment of the present invention, the clearance supporting portion includes a first ball inserting groove formed on the surface of the thrust bearing, a second ball inserting groove formed on the lower end portion of the rotary shaft to face the first ball inserting groove, and balls respectively inserted into the first and second ball inserting grooves.
In the preferred embodiment of the present invention, the ball inserting groove has a v-shaped section and is formed in the center of a surface in which the ball inserting groove is formed.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.